A semiconductor laser, which is used as a light source of an optical communication system and optical measurement system, generally has the following constitution: the semiconductor laser waveguides a light induced and emitted to a junction portion, which is obtained when injecting an electric current to a pn junction semiconductor, by a waveguide structure, and then reflects the light by an optical reflector formed in the waveguide ends to make the light travel reciprocatingly, and subsequently to selectively oscillate a light with a standing wavelength, which is determined in accordance with the length of the waveguide path between the optical reflectors and a refractive index of the inside of the waveguide path.
The semiconductor laser having the above constitution includes a multi-wavelength type without a wavelength selectivity, a fixed wavelength type which has the wavelength selectivity but has a fixed wavelength, and a variable wavelength type which has the wavelength selectivity and can vary the wavelength.
As the variable wavelength type semiconductor laser, there has been known a distributed Bragg reflector (DBR) type semiconductor laser.
FIG. 16 is a sectional side view showing an outline structure of a semiconductor laser 10 having the prior art DBR type optical reflector.
In this semiconductor laser 10, a guide layer 14 having an active layer 13 and a diffraction grating is provided between a p-type clad layer 11, which is formed of for example p-type indium phosphorus (p-InP), and an n-type substrate 12, which is formed of n-type indium phosphorus (n-InP), so as to be continued at a predetermined width and formed of indium gallium arsenic phosphorous (InGaAsP), for example.
In addition, in this semiconductor laser 10, a common electrode 15 is formed over almost the entire surface (element lower surface) of the n-type substrate 12, and a pair of individual electrodes 16 and 17 respectively facing the active layer 13 and the guide layer 14 is formed on the surface (element upper surface) of the p-type clad layer 11.
Note that in this semiconductor laser 10, the n-type substrate 12 in a position near the active layer 13 and guide layer 14 serves as an n-type clad layer 12a for preventing a light from leaking to the outer peripheral part with the p-type clad layer 11.
Moreover, in this semiconductor laser 10, a phase control region may be provided between the active layer 13 and the guide layer 14 in some cases.
Here, when a predetermined electric current Id is injected in between the common electrode 15 and the individual electrode 16, a light in a predetermined band is emitted in the active layer 13, and a part of this emitted light enters a DBR region forming the optical reflector.
Grooves 14a are formed on one surface side of the guide layer 14 at a predetermined interval Λ. In the incident light, an optical component with a wavelength λB determined by the predetermined interval Λ is reflected on the active layer 13 side, while most of the other wavelength components is output from an end surface 10b side (referred to as Bragg reflection).
Therefore, the light with the wavelength λB is selectively amplified between the element end surface 10a and the guide layer 14, and a part of the light is then output from the end surface 10a. 
Here, when an equivalent refractive index of a waveguide path in the inside of the element is represented by n, there has been known that the wavelength λB is represented by the equation:λB=2nΛ.
In addition, there has been known that the equivalent refractive index n is changed depending on a carrier density and temperature of the waveguide path.
An electric current Is injected in between the common electrode 15 and the individual electrode 17 is changed to change the carrier density and temperature of the guide layer 14 to thereby change the equivalent refractive index n of the waveguide path in the inside of the element, whereby it is possible to vary the oscillation wavelength λB.
However, the wavelength change width in a single mode obtained by the variable control in the electric current density and temperature with respect to the guide layer 14 is about 1 percent. When the significant wavelength change width is required, there is a problem that a plurality of semiconductor lasers 10 having the above constitution should be used.
As a technique for solving the above problem, Patent Document 1 describes a tunable laser having a sampled grating structure. In this tunable laser, DBR regions are provided on the front and rear sides of an active layer. The DBR region of the front side is formed such that an interval of a groove for diffraction is gradually changed, while the DBR region of the rear side has a lattice part which has a constant groove interval and is provided at a predetermined interval.
Patent Document 1: PCT international publication <WO 03/012936 A2> (corresponding to Japanese KOHYO Publication No. 2004-537863)